This invention relates to systems and methods of sampling a photodetector and to photocell circuits incorporating the same.
Photodetectors may be used in a wide variety of applications, including imaging applications and optical navigation applications. Recently, photodetector circuits have been incorporated into optical navigation sensors that provide non-mechanical tracking engines for computer mice. The optical navigation sensors employ optical navigation technology that measures changes in position by optically acquiring sequential surface images (frames) and mathematically determining the direction and magnitude of movement. In order to produce a distortion-free image of the scanned surface, the path of the scanning device across the surface should be determined with a high accuracy so that the original image may be mapped faithfully to the scanned pixel data. Such optical navigation systems typically track the scanned path based on detected pixel-to-pixel surface reflectivity differences. These changes in reflectivity may be quite small depending upon the surface medium (e.g., on the order of 6% for white paper). On the other hand, noise signals (e.g., shot noise signals produced by the photodiode), which are amplified by phototransister action, may be relatively large. The photoelement signals in optical navigation sensor applications, therefore, typically are characterized by relatively low signal-to-noise ratios. Consequently, optical navigation sensors typically include circuits that maintain the photoelement signal-to-noise ratios sufficiently high for sensor movement to be tracked. These circuits, however, should be relatively small in size so that a substantial portion of the photocell area may be used for light detection.
Some navigation sensors that have been proposed include two-dimensional photoelement arrays formed on an integrated circuit substrate that includes readout and signal processing circuitry. The photoelement arrays each may have multiple columns and multiple rows of photoelements. Each photoelement may include a phototransistor for generating current in response to detected light signals, servo amplifier circuitry for biasing the base of the phototransistor, and an integration capacitor for storing a charge that is responsive to the amount of photogenerated current. The integrated charge on the capacitor is periodically read and processed so that the amount of light falling on the photoelement during a given reading period may be determined.
The invention features a scheme (systems and methods) for sampling a photodetector that enables the photodetector to be sampled and read concurrently. Thus, the invention may be used to implement a photocell in which image data from the photodetector may be collected during readout. In this way, the invention avoids the substantial non-integration periods that often characterize sampling schemes in which the sampling and readout functions are performed sequentially, especially when operating in a shuttered, non-rolling imaging mode.
In one aspect, the invention features a circuit for sampling a photodetector output node that comprises a multi-integrator circuit. The multi-integrator circuit is coupled between the photodetector output node and a readout node. The multi-integrator circuit comprises multiple storage elements and a control circuit. The control circuit is operable to present at the readout node a first stored sampling of the photodetector output node while concurrently storing a second sampling of the photodetector output node.
Embodiments of the invention may include one or more of the following features.
The multi-integrator circuit preferably comprises multiple integrator circuits. Each integrator circuit may comprise an associated storage node, a respective integration switch coupled between the associated storage node and the photodetector output node, and a respective readout switch coupled between the associated storage node and the readout node.
Control nodes of each of the integration switches may be coupled to a respective input control line. Control nodes of each of the readout switches also may be coupled to a respective readout control line.
In one embodiment, the integration switches and the readout switches of a pair of integrator circuits are cross-coupled so that a respective pair of input control lines control storage of a photodetector output node sampling at one storage node and concurrent presentation of a stored photodetector output node sampling from another storage node. In this embodiment, an output switch may be coupled between the readout node and each of the readout switches. The output switch may have a control node coupled to a readout control line.
In some embodiments, a shutter circuit may be coupled between the integration switches and the photodetector output node. The shutter circuit may comprise a switch that is coupled between the integration switches and has a control node coupled to an integration control line.
In some embodiments, the storage nodes may be operable to be discharged through the photodetector output node. In these embodiments, the storage nodes may be operable to be charged through the readout node.
In another aspect, the invention features a method of sampling a photodetector output node. In accordance with this inventive method, a first stored sampling of the photodetector output node is presented at a readout node while a second sampling of the photodetector output node is concurrently stored.
In another aspect, the invention features a photocell circuit, comprising a photodetector circuit and a multi-integrator circuit. The photodetector circuit has an output node. The multi-integrator circuit is coupled between the photodetector output node and a readout node. The multi-integrator circuit comprises multiple storage elements and a control circuit. The control circuit is operable to present at the readout node a first stored sampling of the photodetector output node while concurrently storing a second sampling of the photodetector output node.
In some embodiments in accordance with this aspect of the invention, the photocell circuit further comprises a plurality of photoelements that are arranged into an array. Each of the photoelements includes a respective photodetector circuit and an associated multi-integrator circuit.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.